The present invention relates to a ceramic substrate with a metallized layer that is typically useful as a substrate for IC (integrated circuit) packages.
Substrates for IC packages, multiple chip mounting substrates, substrates for pressure sensors, etc., are conventionally made of ceramics. The surface of ceramic substrates intended for use in such applications is provided with a metallized layer which assists in the soldering of seal plates or outer lead pins.
With a view to saving energy and simplifying the manufacturing process, it has become a common practice in the production of ceramic substrates to coat a green sheet (a substrate to be fired) with a metallizing ingredient and then form a metallized layer simultaneously with the firing of the green sheet. In this practice of forming a metallized layer simultaneously with the firing of a green sheet, if a ceramic material that can be fired at low temperatures of from 800 to 1,100.degree. C. is used, Au or Cu, or Au-Pt, Au-Pd, Cu-Pt or Cu-Pd alloy that have melting points close to the firing temperature can be used as metallizing components. In particular, Au and Cu have appreciably low resistance to conduction as compared with W, Mo and other metals that are conventionally used in alumina-base ceramics, as ingredients of the metallized layer of a type that is formed simultaneously with the firing of the green sheet. The use of Au and Cu offers advantages such as faster transmission of electrical signals and lower power loss.
However, if common solder, which is made of a tinlead base alloy, is applied to the metallized layer formed of Au or Cu, or Au-Pt, Au-Pd, Cu-Pt or Cu-Pd alloy, the lead or tin component of the solder will get into the metallized layer, which can result in formation of an alloy that has low strength of adhesion to the metallizing component.